ORIGIN OF SPECIES 97 



nous skull, which is pierced by the gullet. The gullet goes 

 straight through the middle of the skull and brain. 



We have said that the vertebrate passes through inverte- 

 brate stages during its early growth, or, in other words, that 

 both vertebrate and invertebrate pass through the same 

 stages up to a certain date. They may in a few words be 

 described as follows : First, the one-celled ovum divides 

 into a "mulberry mass." This mass next becomes a hollow 

 sphere. One side of the ball is then pitted in, so that a cup 

 (the gastrula) is made, lined by a sheet of cells, the endo- 

 derm, covered by a sheet of cells, the ectoderm. But little 

 change is needed to make such an embryo into a sea- 

 anemone. The endoderm is its stomach ; the ectoderm, its 

 body- wall ; the space between them, its body cavity. With 

 a fringe of tentacles round the mouth it is practically com- 

 plete. Now, however great may be the elaboration of this 

 type in invertebrates, its main features remain the same ; 

 the hole left by the pitting in is the mouth ; the nervous 

 system is formed as a circle round this hole. The first 

 difficulty in continuing the line from the invertebrate to the 

 vertebrate sub-kingdom is met with when we try to recog- 

 nise these early stages in the latter. The vertebrate also 

 shows a pitting-in, the "primitive trace" and blastopore, 

 followed by an lip-growth of the "medullary folds," the 

 walls of which grow into the brain and spinal cord. If this 

 corresponds to the pitting-in to form the stomach of the 

 gastrula, it follows that in vertebrates a new stomach has 

 been acquired ; while the old stomach has become the ven- 

 tricles of the brain and the central canal of the spinal cord. 

 How has the new alimentary canal been formed ? Verte- 

 brate embryos (and many invertebrate embryos also) are 

 provided with a store of food -the yolk. For the purpose 

 of tapping this supply of food a diverticulum grows out from 

 the hinder end of the neur-enteric canal. This, according to 

 the view just enunciated, becomes in vertebrates the per- 

 manent alimentary canal. But It has no opening to the 



